Microcombs provide a path to broad-bandwidth integrated frequency combs with low power consumption, which are compatible with wafer-scale fabrication. Yet, electrically-driven, photonic chip-based microcombs are inhibited by the required high threshold power and the frequency agility of the laser for soliton initiation. Here we demonstrate an electrically-driven soliton microcomb by coupling a III–V-material-based (indium phosphide) multiple-longitudinal-mode laser diode chip to a high-Q silicon nitride microresonator fabricated using the photonic Damascene process. The laser diode is self-injection locked to the microresonator, which is accompanied by the narrowing of the laser linewidth, and the simultaneous formation of dissipative Kerr solitons. By tuning the laser diode current, we observe transitions from modulation instability, breather solitons, to single-soliton states. The system operating at an electronically-detectable sub-100-GHz mode spacing requires less than 1 Watt of electrical power, can fit in a volume of ca. 1 cm3, and does not require on-chip filters and heaters, thus simplifying the integrated microcomb.

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Narrow linewidth lasers and optical frequency combs generated with mode-locked lasers have revolutionised optical frequency metrology. The advent of soliton Kerr frequency combs in compact crystalline or integrated ring optical microresonators has opened new horizons in academic research and industrial applications. These combs, as was naturally assumed, however, require narrow linewidth single-frequency pump lasers. We demonstrate that an ordinary costeffective broadband hundreds milliwatts level Fabry-Perot (FP) laser diode, self-injection locked to a microresonator, can be efficiently transformed to a powerful single-frequency ultranarrow linewidth light source with further transformation to a coherent soliton comb oscillator. Our findings pave the way to the most compact and inexpensive highly coherent lasers, frequency comb sources, and comb-based devices for mass production.Kerr optical frequency combs in high-Q optical microresonators 1 are attracting growing interest in recent years 2,3 , especially since the mode-locking via generation of dissipative Kerr solitons (DKS) has been demonstrated on a variety of platforms 4-10 . DKS have enabled compact and broadband low-noise frequency combs with repetition rates in the multi-GHz to the THz domain. DKS have been applied for dual comb spectroscopy 11-13 , coherent communication 14,15 , ultra-fast ranging 16,17 , low-noise microwave master oscillators 8 , calibration of astronomical spectrometers 18,19 and imaging of soliton dynamics 20,21 .Integration of high-Q microresonators suitable for soliton generation has advanced significantly 22-24 . A recent breakthrough was the assembly of an integrated photonics based optical frequency synthesiser 25 pumped with an external III-V/silicon based laser 26 . Yet, most of these demonstrations used single frequency lasers with amplifiers and modulators for soliton generation restricting commercialisation, e.g. highly sensitive wearable spectrometers and ranging sensors.A straightforward approach to obtain DKS in microresonators uses single frequency narrow linewidth tunable lasers for pumping. The frequency of the laser, having a * mg@rqc.ru

We present a novel compact dual-comb source based on a monolithic optical crystalline MgF2 multi-resonator stack. The coherent soliton combs generated in two microresonators of the stack with the repetition rate of 12.1 GHz and difference of 1.62 MHz provided after heterodyning a 300 MHz wide radio-frequency comb. Analogous system can be used for dual-comb spectroscopy, coherent LIDAR applications and massively parallel optical communications.Kerr frequency combs [1, 2] combine unique properties inherent to both narrow-linewidth lasers and broadband light sources. They enable high repetition rates in the multi-GHz to THz domain, broad octave spectrum [3,4], and low-noise RF beat note [5][6][7]. A major advance has been the discovery of the dissipative soliton formation regime in Kerr frequency combs in nonlinear crystalline [8], silica [9], and chip-integrated Si 3 N 4 optical microresonators [10]. This process, that has been demonstrated in a wide variety of microresonator platforms recently, enables fully coherent broadband comb operation and attract significant research interest as possible compact and high repetition rate alternatives to traditional optical frequency combs that revolutionized highprecision spectroscopy [11]. One promising application of frequency combs is dual comb based spectropscopy, a method that enables spectroscopy without the use of diffractive elements. The dual-comb approach [12] allows direct conversion of optical spectra to the radio-frequency domain, and has found applications in such different areas as laser ranging [13] with sub-micron accuracy [14] or high-resolution spectroscopy [15]. In addition the dual comb method can be applied to coherent anti-Stokes Raman spectroscopy (CARS) [16] providing very fast Raman spectrum measurements (∼ 100 µs) and real-time high-resolution spectral imaging. With continued development, the dual-comb spectrometer could replace traditional Fourier spectroscopy in many applications owing to its higher sensitivity [15], fast measurement and stability due to the absence of moving parts. Dual-comb spectroscopy was also demonstrated using quantum cascade lasers (QCLs) in the mid-infrared range (λ ∼ 4 − 9 µm), appropriate for molecular rotational-vibrational absorption spectroscopy [17,18].Recently, dual Kerr frequency comb generation for dual-comb spectroscopy was demonstrated using a pair of optical microresonators in mid- [19] and near-infrared regions [20,21]. In Ref.[20] the dual-comb source consisted of two silicon chip integrated microrings with slightly different radii independently coupled to the same bus waveguide. Integrated microheaters were used for accurate tuning of resonance frequencies of both resonators for combs generation using a single laser as a pump source. In dual comb spectroscopy, the free spectral range (FSR) difference δf between the two generated combs defines the interferogram refresh time ∼ 1/δf and hence signal-to-noise ratio of the radiofrequency spectrum [16]. Well matched resonators having low difference of FSRs would...

Dual-comb interferometry utilizes two optical frequency combs to map the optical field's spectrum to a radio-frequency signal without using moving parts, allowing improved speed and accuracy. However, the method is compounded by the complexity and demanding stability associated with operating multiple laser frequency combs. To overcome these challenges, we demonstrate simultaneous generation of multiple frequency combs from a single optical microresonator and a single continuous-wave laser. Similar to space-division multiplexing, we generate several dissipative Kerr soliton states -circulating solitonic pulses driven by a continuous-wave laser -in different spatial (or polarization) modes of a MgF 2 microresonator. Up to three distinct combs are produced simultaneously, featuring excellent mutual coherence and substantial repetition rate differences, useful for fast acquisition and efficient rejection of soliton intermodulation products. Dual-comb spectroscopy with amplitude and phase retrieval, as well as optical sampling of a breathing soliton, is realised with the free-running system. Compatibility with photonic-integrated resonators could enable the deployment of dual-and triple-combbased methods to applications where they remained impractical with current technology. IntroductionShortly after the inception of the optical frequency comb 2 , it was realised that combining two combs with slightly different repetition rates on a photodetector produces a radio-frequency (RF) interferogram that samples the optical response 3,4 , without any moving parts. Such dual-comb techniques have been demonstrated in both real-time 5,6 and mid-infrared 7 spectroscopy, distance measurements 8 , two-way time transfer 9 , coherent anti-Stokes Raman spectro-imaging 10 , as well as photonic analogue to digital conversion 11 . However, facing the complexity and cost associated with operating two laser frequency combs, novel methods are being actively explored with a view to reduce the system complexity and inherently improve the mutual coherence. For example, instead of phase locking two independent conventional * tobias.kippenberg@epfl.ch mode-locked lasers, both combs can be generated in the same laser cavity 12 , via repetition rate switching of a single comb 13 , or spectrally broadened in the same fibre in opposite propagation directions 14 . As the noise sources are common mode, the relative coherence between the combs is significantly improved, allowing for longer coherent averaging 4 .Recent advances in the field of high-quality-factor microresonators pumped with a continuous wave (CW) laser have led to the discovery of 'Kerr' frequency combs 15 (also termed 'microcombs') that arise due to nonlinear wave mixing mediated by the optical Kerr effect.One particular state of such combs corresponds to the formation of dissipative Kerr solitons 16 (DKSs) -self-localised pulses of light circulating in the resonators arising from the double balance between loss and parametric gain and between dispersion and nonlinearity 17,18 . The ...

Dissipative Kerr solitons have paved the way to broadband and fully coherent optical frequency combs in microresonators. Here, we demonstrate numerically that slow frequency tuning of the pump laser in conjunction with phase or amplitude modulation corresponding to the free spectral range of the microresonator, provides reliable convergence of an initially excited chaotic comb state to a single dissipative Kerr soliton (DKS) state. The efficiency of this approach depends on both frequency tuning speed and modulation depth. The relevance of the proposed method is confirmed experimentally in a MgF₂ microresonator.

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Crystalline optical whispering gallery mode resonators made from alkaline earth fluorides can achieve exceptionally large optical finesse, and are used in a variety of applications, from frequency stabilization and narrow linewidth lasers, to low-noise microwave generation or soliton Kerr frequency combs. Here we demonstrate an efficient coupling method to resonators of these materials, which employs photonic integrated waveguides on a chip based on silicon nitride. By converting a mode from silicon nitride to a free-hanging silica waveguide on a silicon chip, coupling to a crystalline resonator is achieved with a high extinction, while preserving a quality factor exceeding 200 million. This compact, heterogeneous integration of ultra-high Q-factor crystalline resonators with photonic waveguides provides a proof of concept for wafer scale integration and robust, compact packaging for a wide range of applications.